This invention relates generally to an electrostatographic reproduction machine, and more particularly concerns a developer roller assembly and method for making same.
Generally, the process of electrostatographic reproduction includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. A charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed with a development apparatus by bringing a dry or liquid developer material into contact therewith. Two component and single component developer materials are commonly used. A typical two-component dry developer material has magnetic carrier granules with toner particles adhering triobelectrically thereto. A single component dry developer material typically comprises toner particles only.
During development, charged toner, for example, charged dry toner in the form of particles, is attracted to the latent image, thus forming a toner image on the photoconductive member. The toner image is subsequently transferred to a copy sheet. Finally, the toner image is heated and permanently fused to the copy sheet, forming a "hardcopy" of the original image.
As disclosed for example in U.S. Pat. No. 4,267,797, one type of a two-component development apparatus includes a housing defining a chamber for holding two-component dry developer material that includes toner and magnetizable carrier particles or beads. The development apparatus also includes at least a rotatable magnetic roll for transporting a quantity of such developer material in the form of a magnetic brush, into and through a development zone of the apparatus for contacting a latent image to be developed.
As also disclosed for example in U.S. Pat. No. 5,245,392, and U.S. Ser. No. 07/091858 both assigned to the assignee of the present application, another type of a two-component development apparatus is a hybrid development system that includes a housing defining a mixing chamber storing dry developer material consisting of carrier beads and charged toner particles. The development apparatus includes a donor roll mounted within the housing for rotatably transporting charged toner particles from the mixing chamber to a development zone. A plurality of electrode wires are mounted closely spaced relative to the donor roll within the development zone. An AC voltage is applied to the electrode wires for forming a toner cloud in the development zone. Electrostatic fields generated by an adjacent latent image serve to attract charged toner particles from the toner cloud, thus developing the latent image. In addition, a conductive, usually metallic magnetic roll is also mounted within the housing for transporting developer material from the mixing chamber to the donor roll. The magnetic roll is mounted rotatably between the mixing chamber and the donor roll, and serves to magnetically attract and hold magnetizable carrier particles or beads (which have charged toner triboelectrically adhering thereto) onto its roughened or knurled surface. The charged toner is then electrostatically attracted from the carrier beads on the roughened or knurled surface of the magnetic roll onto the donor roll for transporting to the development zone.
The uniformity and quality of latent images developed in the development zone depend significantly on the quantity and uniformity of developer material repeatably transported by the magnetic roll to the development zone in a magnetic brush system, or to the donor roll in a hybrid development system. As disclosed for example in each of the following references, the quantity and uniformity of developer material transported by such a magnetic roll are determined primarily by the surface roughness of the magnetic roll. For example, in this regard U.S. Pat. No. 4,034,709 (issued Jul. 12, 1977 to Fraser et al.) discusses the importance of, and several ways of, roughening the surfaces of magnetic developer rolls. In particular, it discloses such a magnetic developer roll that includes a rough styrene-butadiene surface-coating for holding and directly transporting developer material through a development zone.
Xerox Disclosure Journal (Vol. 4, No. 3 May/June 1979) discloses a magnetic roll in which desired surface roughness is obtained by covering the roll with a netting material such as nylon stockings. Xerox Disclosure Journal (Vol. 4, No. 4 July/August 1979) on the other hand discloses a similar magnetic roll that is roughened by forming a multiplicity of small, shallow depressions in its surface. As a further example, U.S. Pat. No. 4,558,943 (issued Dec. 17, 1985 to Patz) discloses a similar magnetic roll that is roughened by forming valleys in its surface which are then filled with a polymeric material.
As can be expected, when such rolls are used to transport two-component developer material containing carrier beads which can be abrasive, the carrier beads tend to wear out the desired roughness of their surfaces over time. Such wearing out of the surface roughness of a roll disadvantageously and eventually reduces the frictional characteristics of the surface, and hence its ability to repeatably transport desired quantities of developer material. This particular disadvantage is further aggravated in development apparatus that are required to operate at substantially high rates of speed. In such an apparatus, the magnetic roll is accordingly required to rotate at a substantially high number of revolutions per unit time. As can be expected, at such high rates of rotation, centrifugal forces, for example, make it increasingly difficult for the rotating roll to hold onto and transport developer material on its worn out surface. There is therefore a need for an improved magnetic roll with a surface finish that exhibits acceptable developer material holding ability even at high speeds.
Additionally, in a conventional hybrid development system, formation of toner clouds in the development zone ordinarily is due only to the field effects of the biasing electrode wires. It is believed that such field effects can be augmented mechanically by the agitative effects of particular surface finishes of a donor roll as partially exposed in development zone. The result of such augmenting agitation should be more robust toner clouds and improved latent image development.
Conventionally, magnetic and donor rolls have been metallic in order to enable the knurling, for example, of a desired surface roughness thereon. Metallic rolls as such are ordinarily relatively more expensive than non-metallic rolls. Conventional attempts to provide such rolls with significant surface roughness features, for example, by molding, tend to be relatively expensive as well as difficult technically with respect to meeting required dimensions and tolerances. For example, conventional molds for such rolls are made from aluminum or steel, and must be finished and processed in various and expensive ways in order to achieve a desired internal diameter and finish for the desired roll surface roughness feature. The inside diameter and surface finish of the mold must be very accurate and highly polished in order to produce desired roll surface finish. This ordinarily is an inhibitor to the implementation and progress, for example, even of attempts to spincast such rolls in that, as material formulations for a roll change in order to meet various electrical or mechanical requirements as an example, the mold must also change to meet changes in the rate at which the material cures or shrinks. There is therefore a need for a relatively inexpensive and precise method of producing developer roll assemblies with significant surface roughness features for reliable developer material transportation, and augmented mechanical toner cloud formation within a development zone of a hybrid developer apparatus.